Healing after endodontic therapy is monitored by strict criteria based on clinical examination and interpretation of periodic radiographs (see Table 10.1). Clinical follow-up studies show that a large proportion of treated cases with chronic apical periodontitis show signs of healing within 1 year of treatment and in some instances as early as 2–6 months [10, 11] (Fig. 10.2). Some studies have demonstrated that follow-up periods of 4 years or longer may be required when determining success of treatment in some cases. Late periapical changes have been observed radiographically more than 10 years after treatment, whereby healing appeared to have been disturbed and delayed by extension of root canal filling material into the periapical tissues. Small radiolucencies around surplus material should not be misinterpreted as failure [12, 13]. Root-filled teeth should be followed up at least a year after completion to determine whether a favourable outcome has been achieved. Absence of pain, swelling and other symptoms including sinus tract, no loss of function and radiological evidence of a normal periodontal ligament space around the root all indicate success [1].

If radiographic examination reveals the lesion has remained the same size or has diminished but not resolved, then the outcome is considered (uncertain). In this situation the lesion should be assessed until it has either resolved or for a minimum period of 4 years. If a lesion persists after 4 years, then it may be considered to be associated with post-treatment disease and probable failure [1].

During the follow-up period, if the tooth that has been treated exhibits clinical signs and symptoms of infection or a radiologically visible lesion that has either appeared subsequent to treatment or increased in size or if the lesion has remained the same size or diminished but not resolved during the 4-year assessment period, then the treatment can be considered a failure requiring either retreatment or extraction [1].

Scar tissue poses a significant diagnostic dilemma with difficulties to differentiate such a lesion from a radiolucency of pathological origin (apical periodontitis). Clinicians should be aware that although the incidence of this phenomenon is very low, this clinical entity must be taken into account when assessing failure of a treated case. These uncertain cases of healing are demonstrated radiologically by lesions that may have reduced in size but not completely resolved. Provided there are no clinical signs and symptoms, then these types of cases can be monitored for further periods to ensure that no increased lesion size has occurred (which would indicate failure) [1, 14–16].

If root canal treatment has failed, there are usually four possible treatment options, namely, review, retreatment, apical surgery and extraction. Root canal retreatment is often the preferred means of treating a failed root canal treatment, especially when the failure is due to a technical deficiency. The existing root canal filling is removed and the infected root canal system disinfected using irrigants and medicaments. Root canal retreatment is often much more complicated compared to initial treatment, since restorations need to be dismantled and root canal filling materials (including both conventional gutta-percha fillings and unconventional paste fills) have to be removed in order to gain access to the canal system. Iatrogenic errors such as fractured instruments, perforations and canal transportations can further complicate matters. It is imperative to assess that the tooth is restorable prior to embarking on prolonged and often expensive treatment. Longitudinal tooth cracks and fractures that principally occur in the vertical plane or long axis of the crown and/or root can lead to particular uncertainty in terms of diagnostic and treatment decisions. Such cracks and fractures can lead to ongoing symptoms that can often be mistaken for lesions of endodontic origin. Once confirmation of an unfavourable crack or fracture has been made, then extraction will be indicated (Fig. 10.3). If the tooth is deemed unrestorable or unfavourable, then extraction should be considered with suitable replacement if indicated [17].

The decision to gain direct access through an existing restoration or dismantle first must be based on the vulnerability of recontamination during root canal retreatment. It is crucial that the coronal seal be maintained both during and after endodontic treatment to prevent reinfection of the canal space. It is well documented that if coronal leakage occurs (especially when a poor coronal restoration is combined with a poor root filling), then failure is likely to occur [18–20]. If the crown is judged to be of good quality (both clinically and radiographically) with no obvious marginal discrepancies macroscopically, then retreatment can be carried out with a conservative access preparation which can be sealed using either adhesive or amalgam Nayyar core restorations upon completion [21]. In a laboratory study using incisor teeth, cutting an access cavity resulted in a 15 % reduction in relative strength, and subsequent amalgam restoration resulted in a 5 % reduction in relative strength [22]. Restoration of a crowned tooth with a plastic restoration following completion of endodontic treatment ensures that coronal seal is maintained not requiring replacement of the crown.

Dismantling of the coronal restoration is necessary if crown margins are defective or undermined by caries. Retaining such a restoration risks jeopardising further success in the future due to reinfection. For temporary crown and bridge, the restorations can be removed easily using either a scaler or large spoon excavator. Various methods used for permanent crown removal include sectioning, using lifting devices and ultrasonics and can be classified according to whether the method is deemed conservative (prosthesis remains intact), semi-conservative (minor damage to the prosthesis but can potentially be reused) and destructive (prosthesis is damaged and not reusable) [23, 24] (see Table 10.2).

Post removal has been shown to be a predictable procedure using appropriate techniques. Methods for removal of cast, preformed and ceramic posts include ultrasonics and use of post pulling devices (Ruddle post removal kit, Masserann Kit, Eggler post remover, Thomas post remover) [23–31]. Fibre posts may need to be drilled out [32, 33]. Risks of post removal include root fracture [34] and generation of excessive heat causing necrosis of the surrounding bone [35].

Historically numerous materials have been advocated for filling root canals including gutta-percha, gutta-percha carrier devices, synthetic polymer-based polycaprolactone thermoplastic material (Resilon), silver points and endodontic pastes/cements [36] (see also Chap. 7).

Gutta-percha can be removed mechanically using hand or rotary instruments aided by heat or solvents or with the use of ultrasound. Methods employed are dependent on the quality (poorly condensed or well condensed), method of gutta-percha root filling (cold lateral, warm lateral, warm vertical or single cone technique), type of sealer used and personal preference [37–40].

Solvents have been advocated to soften gutta-percha and sealer pastes and assist in its removal. Several solvents have been proposed including chloroform, eucalyptol, rectified turpentine and orange solvent. In general, all solvents are toxic to some degree, and their use should be limited and handled with care [41–45] (see Table 10.3).

Gutta-percha carrier devices such as Thermafil were traditionally comprised of gutta-percha moulded around a plastic or metal carrier. A variety of techniques have been proposed to remove Thermafil obturations including mechanical instrumentation with solvents, heated pluggers and rotary instrumentation. Success of retreatment depended more on the ability to remove the carrier than the technique of gutta-percha removal [46–49]. Recently the manufacturer of Thermafil (Dentsply, Tulsa Dental Specialties, Tulsa, OK) has developed a carrier-based system using a proprietary cross-linked gutta-percha core instead of plastic carriers. GuttaCore can be removed with greater ease than other carrier-based systems. Unlike normal gutta-percha, this core material does not readily dissolve with solvents and is not amenable to plasticising with heat. The manufacturer recommends the use of NiTi instrumentation such as ProTaper retreatment files for the removal of this obturating material [50–52].

Resilon a synthetic polymer-based alternative to gutta-percha can be thermoplastically condensed and combined with a resin-based sealer to create a “monobloc” root filling. Its removal in retreatment is similar to the removal of condensed gutta-percha [53].

The “dirty fills” of endodontic pastes and cements have a poor track record and should be considered obsolete primarily due to their inherent properties of shrinkage, provision of an inadequate seal and serious risk of adverse reactions if inadvertently extruded beyond the confines of the canal system. Non-setting pastes/cements can be easily removed using a combination of solvents, hand and rotary instrumentation and ultrasonics. Hard-setting pastes such as resorcinol-formalin root filling (SPAD, Traitement SPAD, Quetigny, Les Dijon, France) also known as “Russian Red”, due to the colour, sets extremely hard. Tags are formed within the dentinal tubules making retreatment very difficult if not impossible. The use of ultrasound is necessary to remove the hard-setting cement without overzealous preparation or perforation risk compared to the use of burs or drills. The use of Endosolv R (Spécialités Septodont, Saint-Maur, France) is available to aid in the removal of such pastes although clinically its effectiveness is limited with very little effect, if any [54–59].

Silver point fillings although once popular are now considered obsolete. Although ductile enabling easy placement and radiographically appealing, these fillings resulted in voids due to cross-sectional round silver points attempting to obturate a three-dimensional root canal system relying on a sea of sealer to plug the gaps. Over time sealer dissolution and silver point corrosion products as a result of tissue-fluid contact resulted in failure and their ultimate demise. Retreatment cases occasionally require the removal of silver point fillings. Successful removal of silver points is dependent on the ability to expose the silver point during disassembly without avoiding further fracture, loosening the silver point from the surrounding cement casing and gaining an adequate purchase in order to extract it from the root canal. If the head of the silver point has been exposed correctly and not fractured at the level of the root canal, then a surgical haemostat, artery forceps or Steiglitz forceps can be used to grasp the silver point and aiding removal. If the silver point is flushed with the root canal orifice, then the use of ultrasonics, Masserann Kit (Micro-Mega, Besancon, France) or Cancellier Kit (SybronEndo, Orange, CA, USA) may be advantageous in trephining and engaging the silver point for removal purposes [60–63].

Chemomechanical preparation and disinfection protocols used for the management of primary untreated cases can be applied to previously root-treated teeth. Multi-visit appointments with the use of intra-canal inter-appointment medicaments are mandatory since the time to properly disinfect the root canal system in a single visit is often impossible. Dismantling of existing restorations and previous root-filling materials and renegotiating canals to length are time-consuming procedures requiring proper disinfection procedures to be postponed until the second appointment [64].

The outcome of endodontic treatment is generally better than root canal retreatment, and this difference may be attributed to both the nature and location of the infection and treatment difficulties encountered when trying to rectify previous treatment errors. A systematic review of 17 studies published between 1961 and 2005 reported that the pooled success rates of root canal retreatment ranged between 70 and 80 % [65]. A systematic review of 63 studies published between1922 to 2002 reported the pooled success rate of root canal treatment ranged between 68 and 85 % when strict criteria were used [66]. The major difference between the outcomes of primary and secondary root canal retreatment resides only in the ability to predictably access and negotiate the root canal system to the (residual) apical infection. Outcome data offers a very favourable prognosis for non-surgical root canal retreatment provided it is performed to acceptable guideline standards [67].

A few studies have compared non-surgical retreatment and endodontic surgery. A Cochrane review concluded that the short-term healing rates might be higher in the surgically treated cases [68]. The two randomised controlled trials comparing outcome of non-surgical retreatment vs. surgery hypothesised that the surgical treatment resulted in a more rapid bone fill but was associated with higher risk of “late failures” [69, 70]. The most recent meta-analysis comparing the outcomes of non-surgical root canal retreatment and endodontic microsurgery concluded that the latter was a reliable treatment option with favourable initial healing and a predictable outcome [71]. Bearing in mind that the cause of failure is usually residual infection within the root canal system, then non-surgical root canal retreatment should be the treatment of first choice. In cases where apical anatomy has been altered (canal transportation or blockage), whereby rendering re-treatment procedures ineffective or impossible, then surgery may be the only alternative treatment option. An informed decision by the patient should be reached based on careful case assessment and known prognostic factors that may influence treatment outcomes. One must also take into consideration the estimated cost-benefit analysis for all options including extraction and replacement when considering any treatment. A typical example is an anterior tooth with a well-fitting cast post and core restoration where dismantling is not only a costly exercise but also carries the risk of potential root fracture. Such cases may be more amenable to a surgical approach rather than non-surgical retreatment.